Electron beam microcolumns based on microfabricated electron optical components and field emission sources operating under the scanning tunneling microscope (STM) aided alignment principle were first introduced in the late 1980s. Electron beam microcolumns are used to form a finely focused electron beam. See Chang, T. et al., "Electron-Beam Microcolumns for Lithography and Related Applications" J. Vac. Sci. Technology, B 14(6), pp. 3774-3781, November/December 1996, and Lee, K. et al, "High Aspect Ratio Aligned Multilayer Microstructure Fabrication" J. Vac. Sci. Technology, B 12(6), pp. 3425-3430, November/December 1994, incorporated by reference herein. These columns offer the advantages of extremely high resolution with improved beam current, small physical size, and low cost, and can be used in a wide variety of applications, such as electron beam lithography.
Microcolumns are high-aspect-ratio micromechanical structures including microlenses and deflectors. The microlenses are multilayers of silicon chips (with membrane windows for the lens electrodes) or silicon membranes spaced apart by 100-500 .mu.m thick insulating layers. The lenses have bore diameters that vary from a few to several hundred .mu.m. For optimum performance, the roundness and edge acuity of the bores are required to be in the nanometer range and alignment accuracy between components on the order of less than 1 .mu.m. A microlens consists of a plurality of microlens components or elements, accurately aligned, for the purpose of focusing electronics. A microcolumn consists of one or more microlenses together with other components, such as deflections and cathodes, all aligned together.
Electrodes of the microlenses can be made from 1 to 2.5 .mu.m thick silicon membranes by electron-beam lithography and reactive-ion etching (RIE). The starting material is, e.g., a 4 inch diameter and 500 .mu.m thick double-sided polished wafer containing a plurality of 7 mm.times.7 mm die. At the center of each die is a 1 mm.times.1 mm membrane formed by wet isotropic etching using, e.g., either a highly boron doped or a reverse-biased p/n junction etch stop.
FIG. 1 shows a cross-sectional view of a 1 kV microcolumn based on the well-known STM aligned field emission (SAFE) concept, showing source lens section 1 and Einzel lens section 3. Scanning tunneling microscope (STM) scanner 5 emits an electron beam 6 in the direction of sample plane 25. The beam 6 first passes through the source lens 1, composed of silicon apertures, 5 .mu.m diameter extractor 7, 100 .mu.m diameter anode 11, and 2.5 .mu.m diameter limiting aperture 13. The three apertures are separated by two insulating spacers 9. The insulating spacers 9 are typically formed of a heat-resistant borosilicate glass, commonly known as Pyrex, but could be made of any other suitable insulator, such as SD-2 glass made by Hoya. The source lens 1 is mounted on aluminum mounting base 15, which contains an octupole scanner/stigmator 17. The electron beam 6 then passes through the Einzel lens 3, which is composed of two 100-200 .mu.m diameter silicon apertures 19 and 23 with a 1-1.5 .mu.m thick free-standing silicon membrane aperture 21 disposed therebetween. Each silicon layer is again separated by insulating spacers 9.
The source lens 1 and Einzel lens 3 are shown expanded and in greater detail in FIGS. 2(a)-(b) with similar reference numbers identifying the same structures.
The conventional approach to bonding the insulating and microlens layers of the microcolumn involves the use of anodic bonding. Anodic bonding is an electrochemical process for heat sealing of glass to metal and semiconductors. At elevated temperatures (300-600.degree. C.), Na.sub.2 O in the Pyrex or other glass dissociates to form sodium and oxygen ions. By applying a potential between a first silicon layer and a glass insulation layer, sodium ions in the glass migrate from the silicon-glass interface, while uncompensated oxygen anions move toward the induced positive charge of the silicon anode to form chemical bonds.
Anodic bonding disadvantageously must be conducted at elevated temperatures, which typically requires several hours of heat-up (to approximately 400.degree. C.) and cool-down time, as well as a physical connection of a high voltage probe, during which time drift, bond-induced shift, and expansion can degrade the alignment. This process must then be repeated for each additional layer.
Assembly of the lenses and the column typically involves stacking together silicon components and borosilicate glass spacers and using anodic bonding to bond each layer to the microcolumn. Because the apertures in the microlenses must be precisely aligned, assembly of the microcolumn is complex and time-consuming. One assembly method requires that each lens be aligned under an optical microscope and that they be anodically bonded one layer at a time.
Accordingly, there is a clear need for a method of forming microcolumn structures that simplifies the assembly process by enabling the precise alignment of microlens components to be carried out quickly and easily.